So far, a liquid crystal flat-panel display has been the one capable of comprehensively catching up with and surpassing color cathode ray tubes (CRT) in terms of comprehensive performance, such as brightness, contrast, power consumption, life, size and weight. It has become a mainstream product in the image display technical field nowadays due to its excellent characteristics, such as excellent performance, large-scale production, low cost of raw materials and broad development space. The basic operation principle of an existing liquid crystal display device is to adjust the liquid crystal transmittance of the backlight by applying a data voltage including image information to the liquid crystal and changing the twist degree of the liquid crystal, such that expected image display is realized. To obtain the image display with vivid color, besides controlling the color mixing quantity of red-green-blue primary color for each pixel point (the smallest area unit of image display), a display driver circuit of the liquid crystal display device also needs to implement fine adjustment on the brightness level of each pixel point, i.e., gray scale adjustment. The larger the quantity of gray scales is, the finer the effect of presented picture is. At present, particularly for the liquid crystal flat-panel display, the common gray scale adjustment approach is a digital-typed voltage-gray scale adjustment. In this approach, the quantity of the gray scales is determined by bits of an image data signal. Taking the 8 bit liquid crystal flat-panel display as an example, the image data signal is 8 bit, and 28=256 gray scales may be presented. Further, for color displaying, as colors of each pixel point is constituted of red-green-blue primary color, the color change of each pixel point is thereby substantially caused by the gray scale change of red, green and blue sub pixels constituting the pixel point, such that a number of 28×28×28, about 16.70 million, colors (chromatic number) may be presented. This means that when larger chromatic number is expected for achieving the better picture display effect, the bits of the image data signal need to be increased, and correspondingly, the quantity of the gray scales needs to be increased. In specific circuit implementation, a voltage signal for adjusting the gray scales (the value of which is merely a few volts) needs to be divided more finely, which undoubtedly increases certain difficulty in the design and manufacture of the display driver circuit. The most direct influence by this is that, when the bit of the image data signal is increased by one bit, the quantity of circuit elements in a data driver circuit for adjusting the gray scales in the display driver circuit needs to be doubled. This may lead to a series of problems, for example, the size of a chip is increased to improve the picture display quality and the investment cost is thereby raised.